The present invention relates to rotary fluid pressure operated devices, and more particularly, to such devices which include an internal gear set, two relatively rotatable housing portions, and a shaft member for transmitting torque therebetween.
Although it should become apparent from the subsequent description of the present invention that it may be useful with many types of configurations of fluid pressure devices, the invention is especially advantageous when used in a wheel motor, and will be described in connection therewith.
Also, although the invention may be used with devices having various types of internal gear sets, such as those of the crescent type, the invention is especially adapted for use in a device including a gerotor gear set, and will be described in connection therewith.
Furthermore, although the invention may be used in devices having various configurations of commutating valving, such as rotating disc valves, it is especially suited for use in devices having cylindrical spool valves, and will be described in connection therewith.
Fluid pressure operated wheel motors of the type utilizing a gerotor displacement mechanism to convert fluid pressure into a rotary output have now become well known, and are especially suited for low speed, high torque applications. In most of the known wheel motor designs of this type, one of the primary factors limiting the torque output capability of the motor is the strength of the torque transmitting connection between the stationary portion and the rotating portion. Typically, this drive connection comprises a set of internal splines defined by the gerotor star, a set of internal splines defined by the stationary housing portion, and a main drive shaft (dogbone) having a set of external splines at each end thereof, in engagement with the sets of internal splines. Generally, the internal splines are straight, whereas the external splines are crowned to take into account the angle at which the drive shaft is oriented relative to the axis of rotation of the motor. Therefore, although the invention may be used with devices in which the externally toothed star member merely rotates about its axis, and the dogbone merely rotates about its axis, the invention is especially advantageous when used in a device in which the star member orbits relative to the internally toothed ring member, and the dogbone nutates or wobbles, and the invention will be described in connection therewith.
One of the primary reasons for the limited torque capability of prior art wheel motors is the heat buildup which occurs as a result of the engagement between the internal and external splines. The heat buildup problem is worsened in wheel motors wherein the gerotor ring rotates, and the dogbone must prevent rotation of the gerotor star, relative to the stationary housing portion, permitting only orbital movement of the gerotor star. The result is a continual rubbing movement of the external splines against the internal splines, which causes substantial frictional heat.
In prior art wheel motors, the internal-external spline connections have not had sufficient lubrication. One reason is that they are frequently located in the end of a blind bore, so that any lubricating fluid which is leaked into the bore is likely to remain stagnant around the spline connection, rather than transmitting heat and contamination particles away from the spline connection.
The problem of insufficient lubrication of the spline connections becomes especially serious when the motor is operating at relatively low speeds (e.g., in the range of 5 to 10 rpm), and at high output torque (e.g., 2000 in. lbs.). Under these conditions, the temperature of the spline connection rises, the viscosity of the lubricating fluid drops, and a "break-through" of the oil film may occur, resulting in metal-to-metal contact of the splines. This, in turn, causes even more heat buildup, a further decrease in torque capacity, and possibly, eventual failure of the spline connection.